The present invention relates to a timing data reproduction system for a digital receiver.
In digital communications utilizing frequency shift keying (FSK), only the digital signal is transmitted so that the timing data required for sampling the incoming data signal must be generated in a receiver so as to decode the incoming data signal correctly.
To this end, there has been devised and demonstrated a system using a pilot tone signal. That is, when a telephone circuit or line is used for data transmission, the carrier is modulated in a suitable manner so as to transmit the data. A pilot tone is transmitted on an idle frequency band which is not used for the data transmission. At the receiving side, in response to this pilot tone the timing data for sampling is received.
Since the data and the pilot tone are sent by different frequency bands, in addition to a data receiver an additional receiver for receiving the pilot tone must be provided. Thus the pilot tone system is disadvantageous in that it is very complicated in construction.
There has also been devised and demonstrated a system wherein the zero-crossings of the received base-band signal are directly detected or the transition points of the two-valued signal obtained by the reshaping of the received signal so as to control the frequency or phase of a local oscillator which generates the sampling timing clock, thereby synchronizing the sampling clock with the incoming data signal. This system has also some problems as will be described below.
In the zero-crossing detection system, all of the signals to be processed cannot be digitized because the analog signal must be used for the zero-crossing detection. In the system wherein the transitions of the two-valued signal, the differentiation of the signal is needed for the direct transition detection. In order to overcome this problem, there may be considered to detect the transition time points by means of the digital sampling, but this is possible only at a considerably high sampling frequency. Furthermore, in the case of the ultra-high speed data transmission through for instance an optical transmission line, it would be extremely difficult to obtain directly the time relationship between the zero-crossings and the sampling time points. Even in the case of the low-speed data transmission through telephone networks with FSK modes, the incoming data signal must be sampled at a frequency considerably higher than the baud rate of the incoming data signal in order to detect the baud timing data. If it is attemped to process all these operations with the use of a microcomputer in full-digital manner, the overhead of the microcomputer would be inhibitively increased. For instance, assume that the transmission speed be 600 bauds and 16 samplings be made for one baud. Then one sampling must be executed in such a short time as 1/(600.times.16).apprxeq.104.times.10.sup.-6 seconds. Such high sampling speed surpasses even a highest processing speed of the microcomputer.
There is also known a timing data reproduction system of the type using a squaring circuit and a narrow-band filter. However the narrow-band filter must have a high Q and must be correctly tuned to the frequency of a predetermined baud rate. When the Q factor is too low, the reproduced timing signal will be considerably adversely affected by jitter. When the narrow-band filter is not correctly tuned, off-set of the phase of the reproduced timing signal will occur, resulting in phase error. In addition, in order to attain in the form of an analog circuit a narrow-band filter which has a high Q factor and is correctly tuned, the timing data reproduction system needs component parts which must operate at an extremely higher degree of accuracy in an extremely highly reliable and dependable manner. In addition, the system would require a large number of ALUs, thus increasing the cost.